Edison denies a claim that it knew of problems with steam generators before they were installed at the San Onofre plant.

Southern California Edison, the operator of the troubled San Onofre nuclear power plant, pushed back against two federal lawmakers who said that the utility company was aware of defects in the plant's replacement steam generators before they were installed.

Sen. Barbara Boxer (D-Calif.) and Rep. Edward J. Markey (D-Mass.) sent a letter to the U.S. Nuclear Regulatory Commission this week saying that a leaked report from steam generator manufacturer Mitsubishi Heavy Industries showed that Edison and Mitsubishi knew of problems with the design.

Boxer and Markey, citing a report that has not been made public, said the utility giant and the manufacturer "rejected enhanced safety modifications and avoided triggering a more rigorous license amendment and safety review process."

Edison responded Thursday by denying the allegations and saying the lawmakers had taken "selective comments" from a lengthy report:

"It is simply not accurate to suggest, as the letter does, that when they were installed SCE and MHI were aware of serious problems with the design of San Onofre nuclear plant's steam generators," the company wrote in a statement.

"Indeed, MHI, the manufacturer of the steam generators, warranted the steam generators to be free from defects for 20 years after installation. SCE would never, and did not, install steam generators that it believed would not perform safely."

Neither the lawmakers nor Mitsubishi and Edison have elaborated on what changes the companies chose not to make.

The plant has been shut down for just over a year because of unusual wear on steam generator tubes that carry radioactive water. One tube leaked in January 2012, releasing a small amount of radioactive steam.

Edison and San Diego Gas & Electric — which owns a 20% share in the plant — spent a combined $771 million replacing the steam generators, which ratepayers are now repaying.

Representatives of both companies, as well as of several consultants involved in the investigation of San Onofre's problems, gave a briefing Thursday to the five-member governing board of the NRC.

The meeting, scheduled before Boxer and Markey's letter, was the first time since the plant's shutdown that the full commission has questioned Edison and Mitsubishi. But the discussion steered clear of the allegations in the letter and focused on the technical causes of the tube wear.

Steam generator tube degradation has long been an issue in the nuclear industry, although it has become less prevalent since the majority of plants switched to a more corrosion-resistant tube material. In 1976, there were 28 forced outages at U.S. nuclear plants because of tube leaks, according to the Electric Power Research Institute. Before the leak at San Onofre, there had been no such incidents in six years.

An NRC investigation concluded that Mitsubishi's computer code failed to predict thermal-hydraulic conditions — high-velocity, dry steam flowing around the tubes — that caused the tubes to vibrate excessively and rub against each other and against support structures.

The phenomenon, known as fluid elastic instability, has occurred at other plants, but the tubes at San Onofre showed a new pattern of vibration, leading to rapid wear in one of the plant's two reactor units.

Edison is proposing to restart the other unit — which had less-severe damage because of differences in the support structures — at partial power. The NRC has not made a decision on that plan.

Dan Hirsch, a lecturer at UC Santa Cruz, was the lone critic of the nuclear industry who made a presentation at the meeting. Hirsch and his students put together a report showing that both units at San Onofre had more worn tubes than all but one other similar plant in the U.S. with recently replaced steam generators.

"San Onofre in just one or two years has experienced more damage than most steam generators do in decades," Hirsch said.

Ken Karwoski, a senior advisor with the NRC's division of engineering, said the depth of the wear determines the safety significance, not the number of tubes with wear. Unit 2, which is proposed for restart, had nearly as many tubes with wear as the other unit — about 1,600 and 1,800, respectively — but the wear in Unit 2 was shallower.

The NRC has scheduled a public meeting to discuss the status of its San Onofre review Tuesday from 6 to 9 p.m. at Capo Beach Church in Capistrano Beach.